Elevated Bridge B/C

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Re: Elevated Bridge B/C

Postby dragonfly » August 27th, 2009, 6:25 pm

Designing by blind trial and error is weak, time consuming, and uninstructive. I'd recommend doing as Jim suggests, testing your work against the jhu applet or a similar static analysis program and making adjustments.
Hm....I have to say, those are three very strong adjectives that I, for the most part, disagree with. While initially trial and error may be a bit time consuming, 'blind' is not the correct way of describing how we build. Simply building a ton of bridges won't get you anywhere, but building a lot of bridges to test intriguing ideas is the only true way to achieve the answer you're looking for. I believe that Nejanimb was trying to say that while programs, of any sort for that matter, are indeed useful in some simpler cases, if you understand the general idea of how the forces would work in a design whose loads are distributed more complexly, that the best way to test your theory is to take a couple hours and get it over with :D That way, you will end up with a straight up answer by the remains (or lack thereof) of your bridge!
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Re: Elevated Bridge B/C

Postby smartkid222 » August 27th, 2009, 6:47 pm

basicly, i've built tons of bridges, towers, etc., never used a program or calculated forces but was still good enough to medal. Obviously i see that this is not the correct method but it still works to an extent; i think that this is what they were trying to say. As a result of all this talk of always with stresses and that program keep coming up, this is one thing i hope to change this year. I always built stuff, but never really learned the physics/math behind it.
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Re: Elevated Bridge B/C

Postby Aia » August 27th, 2009, 7:36 pm

I don't see any faults with trial and error as long as you test new ideas methodically and carefully. While the ability to calculate forces on the bridge would be valuable, it's a very difficult proposition for most teams. I looked at several websites last year that would allow you to build virtual trusses, and most were fairly limited or only calculated forces for basic trusses. After I couldn't find any websites to help, I asked several physics teachers to help me calculate forces. They were unable to calculate the forces, and interpreted my current bridge in same way I did by looking at which pieces were in tension and compression, how the load transferred, etc. Simply put, calculating truss forces isn't feasible for most teams, especially if one considers the limitations of these truss programs and the countless variations in design this year. Unless you are a physics wizard or have a helpful contact, calculating these forces are exceptionally difficult.

I would also like to point out the largest virtue of trial and error. From personal experience, I know that every single tower, boomilever, and bridge I've built has taught me something new, whether it was knowledge about that particular design or an improvement in building technique. Even as I built bridges this year, I recalled knowledge from past towers or boomilevers that I tested, and was able to synthesize my research to build better structures.
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Re: Elevated Bridge B/C

Postby blue cobra » August 27th, 2009, 8:48 pm

I think programs such as the JHU one are most useful for general ideas. For example, If you were thinking of including a certain type of truss in your bridge, you could draw that and see a basic idea of which members would be in compression and which ones would be in tension, as well as how much relative to other peices. You could decide which members probably need more or less strength, and what kind. A lot of this can be instantly deduced by an experienced bridge-builder, but having the numbers would surely help. Being able to calculate the forces wold be fantastic, but, especially for students, learning how to do those calculations could be more difficult and time consuming than building numerous bridges. Plus, you get building experience and a definitive answer. Calculating stresses are certainly best from an engineering standpoint, but when you test your design in real wood in a few hours, people prefer to do that.

Anywho... lap joints are stronger than butt joints. I don't quite understand exactly why they are much stronger. It just seems a force on one of the pieces in a lap joint would pull the joint apart, while in a butt joint a force applied to a member on top would push against the other member and not pull the joint apart.
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Re: Elevated Bridge B/C

Postby nejanimb » August 27th, 2009, 9:25 pm

Truly "blind" trial and error would definitely be time-consuming, but I don't think uninstructive. On the contrary, like Dragonfly said, I think to put a design in wood and get it through its paces on the blocks is the only way to really see what's going on. I agree with Aia - we built close to 40 bridges this season as a team, and we learned something from every single one, all the way down to the bridges built in the week between states and nationals.

Since we have a number of people building bridges, what we did to start the season was to let everyone try whatever they wanted based on what they knew from past experience and their understanding of the physics involved. We'd all get together to watch tests, give critiques, and talk about what we could learn from the bridge and how it loaded. It took us a couple months, but eventually we hit upon a good design. Then, we spent the rest of the year perfecting it. We tried changing one variable at a time - one person would try using a different cross-section for the main compressions while another person would try changing the angle of the legs while another person would try tapering the sides together, etc. We'd regroup, watch the tests, talk about what we found, and then move on to a new round. One person would try .15g for the compressions while another tried .21. Rinse and repeat.

The point of this description: we tried consulting the truss-bulider program, but it couldn't actually teach us how to build a successful bridge. It might show some simple versions of sturdy triangle arrangements, and it can show relative loads... but how can you make that work in balsa wood? In the end, building 30+ bridges taught us a TON during the course of this past season - what works, what doesn't, and I know more for it. Not only do I know more about that each particular design, but I learned things about engineering in general and how structures and forces work. I was able to see in a very tactile, real way laid out in front of me in splintered balsa fragments what I should do next time and how we could improve. This was infinitely more real and useful to me than a numbers readout from a program that really doesn't teach me anything about how to make the next bridge better than the one we just broke. Without trial and error, how can you improve? Build one bridge, and then another, but learn nothing in between from the trial and error process? That seems insane to me. It's an absolutely essential part of the design process, to my way of thinking, and one that cannot be replaced by working with fancy spreadsheets or stress-analysis programs. For us high schoolers, this is critical to learning about how things work. It's building something, trying it out, and improving our design and methods that's really instructional - not just simply heeding the advice of a structural engineer we know or a statics program we couldn't emulate ourselves or a spreadsheet that we didn't write ourselves. To me, that would be far more "blind" than being methodical with trial and error, like we were.

Seems to work too: like I said before, none of the top bridges (the C bridges, at least) would have held up to that JHU app, but clearly they were successful nonetheless. Our bridge sits proudly atop my desk, and I'm happy when I look at it and see each individual piece of it, and I can remember the bridge that we built last season that taught me the lesson that resulted in it being exactly how it is now. I see "oh yeah, that gusset was added after it broke such a way" and "oh we moved that there after we almost hit minimum specs" and "we added that extra piece after it kept breaking at the top" and "we decided on .19g for that part since the ones heavier held everything and the ones lighter broke early" and so and and so forth. The final product was absolutely a result of a full year's hard work, with things learned at every step of the way through rigorous trial and error. While I may not fully understand static analysis, I don't think that means I know nothing about structural engineering, nor do I think that my method was weak. Above all though, I know it was as far as you get from uninstructive.

Aside from that rant, rjm, the clarification about how that statics program doesn't show deflection makes a lot of sense to me. I knew that was the case, but I didn't put it all together to come to that conclusion properly - thanks. That makes my misunderstanding of the inverted V and the rest of the issues much more clear - I get that all now. Thanks!
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Re: Elevated Bridge B/C

Postby andrewwski » August 27th, 2009, 9:41 pm

After I couldn't find any websites to help, I asked several physics teachers to help me calculate forces. They were unable to calculate the forces, and interpreted my current bridge in same way I did by looking at which pieces were in tension and compression, how the load transferred, etc. Simply put, calculating truss forces isn't feasible for most teams, especially if one considers the limitations of these truss programs and the countless variations in design this year. Unless you are a physics wizard or have a helpful contact, calculating these forces are exceptionally difficult.
Really? It's not that hard to calculate them all by hand...it's all vectors and moments of inertia. We calculated them many times in my Principles Of Engineering class. Yes, it's a pain in the neck and very time consuming, but it can be done. I'm surprised that your physics teachers couldn't do it.

Designing a decent bridge without analyzing the forces involved is certainly possible, but much more difficult. Without knowing the forces in each member and joint, it makes construction more like guesswork, and analyzing the failure is done with much less certainty. Having an idea of what would happen under ideal conditions at least gives you some understanding of why something happened. Not that it can't be done without that information, but it's a lot more trial and error than needed.

Now, it is important to build, break, and learn from there when rebuilding. I only made one bridge design this year on paper, but by building, breaking, analyzing, and rebuilding, was able to get about 2.5 times the efficiency the original bridge had. But I used the truss calculations extensively in my analyzable. If you were to look at my first and last bridges, they look very similar, however, little things such as a particular member size and stuff can make a huge difference.

I don't think I would have necessarily had a bridge competitive at the national level (my efficiency peaked out at just under 1200) but I certainly was able to take a design and improve on it, and understand why it worked the way it did.

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Re: Elevated Bridge B/C

Postby dragonfly » August 28th, 2009, 4:35 am

I think another thing that was crucial to our success this year was the countless changes that we made to our design. If anyone saw our original bridges, they would have no way of knowing what our nationals bridge looked like. While possibly if we'd been concentrated on one simpler design like Andrewwski, (possibly for one of those original bridges) that may have assisted improving that design, making a better, and essentially different, bridge, required changes that a program could not have told us to make.
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Re: Elevated Bridge B/C

Postby JimY » August 29th, 2009, 6:50 am

We certainly have two distinct camps here. Both are valid ways of working on the problem. Just from a time efficiency viewpoint, designing via programs is much much faster. In a post from last year, I mentioned that both our B and C division bridges were designed in October and not changed after that. If our high school team would have made it to nationals, it would have placed 3rd in the event with no changes past state, so we may have slipped closer to first there with a bit more work. It was a series of 7 interconnected triangles, just like the B division winner. For B, we started the season with efficiencies just over 2000 and ended up at nats at just over 2500. For C, we started the season with efficiencies of about 1600 and ended up at just under 2200.

Designing via a program allows you to make small tweaks in the position of the nodes and see the effect on the stresses of the pieces in just seconds. For example, one node in the B division design was moved around such that one of the chords had a slight amount of tension on it without significantly changing the load on the other chords connected to the same nodes. This allowed a smaller piece to be used than if the piece were in compression. You can get that from trial and error, but at 5-7 hours per build, why spend that much time when seconds will do?

After the design was finalized, the trial and error work began. It was just a matter of how little mass could we get away with and not have it fail too early in the loading. This took something on the order of 10 iterations for the B division winner at nats. We did about 6 iterations for the C division state design above. We tried using significantly more balsa for both divisions, for example, but rejected it for both divisions.

Back in the 2000 season (coaching B only), I was firmly in the trail and error camp for the event. This was because I hadn't really though about using a program to help. Then one of the other parents asked to take one of our designs to his work and put it into his $15000 program that also calculates deflections on pieces (he was part owner in a company that builds roads and bridges). The programming idea stuck, and I found the website given in my previous post on this thread. So, after that season concluded, I decided to put together my own structural design program in Excel (which uses only high school level statics from Physics class) and calculated the stresses on all the trusses that could be calculated on what we built during that season and compared this to where we thought failure occurred. It was a true breakthrough. This allowed our workload in the event to go way down. Despite spending far less time working on the event, we ended up similar results. So, my heart goes out to all the teams that do this event by trial and error because we were there at one point too. Just getting through that paradigm may be very tough, since you are not engineers after all. However, if you manage to get through it, the other side is like a huge breath of fresh air. If you're planning on an engineering career, I strongly suggest adding a truss program to your repertoire for the event. It could be part of your legacy for your team, as it can be used for years to come.

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Re: Elevated Bridge B/C

Postby baker » August 29th, 2009, 7:45 am

JimY, How are you deteriming what a piece of balsa at a given dimension is capable of in compression or tension? Yes a piece of steel is consistant, but no two pieces of wood, even cut from the same parent piece are the same. So I wonder how a program helps with unknowns, or do you do a zillion of tests on different sizes both tension and compression?

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Re: Elevated Bridge B/C

Postby jander14indoor » August 30th, 2009, 4:54 am

Wood selection is a KEY element of this event! This is true whether you are using a trial and error approach or a analytical one. See last years discussion and search using "wood selection" as your key words and you will quickly find several in depth discussions on the topic.

For the analytical approach, yes wood varies, but over a limited range. The analysis tells you what you need, and then you select the wood to meet the requirements. This is NO different than in real life engineering! The end criteria is different, but the approach is the same.

Example, ceiling trusses. These are commonly made of wood and by code have to meet certain load requirements. In order to stay in business, companies use analytical programs to minimize the wood used to save costs, and have to deal with wood variability in doing so.

For the trial and error approach, this is just as important. Unless you select wood for consistent properties, how do you get a design to repeat its performance from one trial to the next? If you can't do that, you are just shooting in the dark and making design changes based on random changes in wood behavior, not truss layout.

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